What do pronouns do in sentences?

Pronouns are used instead of nouns. Words like he/ it/ you/ me/ him/ their/ your are pronouns.

For example:

The dog chased the cat and then the dog sat down because the dog was exhausted.

Instead of repeating the dog, the dog, in this sentence we can use a pronoun.

The dog chased the cat and then he sat down because he was exhausted.